Gear vibration and clash in an internal combustion engine can lead to intense whining, sharp impulse noise (e.g., rattle), or both. These noises can cause extreme operator and passenger discomfort in a vehicle. Engine whine and rattle also add to the constant cacophony that can make proximity to transportation routes unpleasant. Because of this, performance standards and environmental regulations relating to vehicles increasingly include NVH limits.
Backlash is a gap (e.g., lash) typically present in meshing gears to account for manufacturing tolerances, to prevent binding of gears at higher operating temperatures, and to account for other variations in gears that can exist during operation of an engine. In gear trains of opposed-piston engines, during torque reversals, the driving gear makes contact with both flanks of the corresponding driven gear in a mesh, thereby producing gear rattle, especially when there is excessive backlash in the system.
The gear train of an opposed-piston engine with dual crankshafts inherently experiences events where contact is lost between teeth of adjacent gears in a mesh that produce rattle and vibration. For example, in the case where a phase difference is provided between the crankshafts (i.e., crank lead) in order to differentiate port opening and closing times, the gear train is subjected to a torque reversal event at least once every cycle of engine operation. Even without an inter-crankshaft phase difference, momentary inter-gear torque reversals can result from any of idler bounce, gear rotational distortion, shaft rotational distortion, or a combination thereof. Torque reversals result in gear train rattle when gear backlash is present.
Backlash control in an opposed-piston engine can be a balance between noise control, minimization of friction loss, and the efficient transfer of torque in the gear train. Conventional backlash controls include methods and apparatus that fix the backlash of an engine prior to operation of the engine (e.g., movement of idler gears, the use of select fit gears), or perhaps additionally after operation and alteration of an engine, but dynamic backlash control is not a feature of these conventional controls. Scissor gears of the conventional type, such as described in U.S. Pat. No. 5,979,259, are pre-tensioned with one or more torsional springs to take up the lash in a gear mesh between the scissor gear and an adjacent gear.